How to convert date to timestamp in PHP: complete guide

Hello everyone! I'm a beginner PHP developer and I've encountered a problem when working with dates. I need to convert a date string to a Unix timestamp (number of seconds since January 1, 1970) for storing in a database and subsequent date comparison.\n\nThe problem is that I have a date in the format "2023-12-25 14:30:00", and I need to get a timestamp to check if a subscription has expired. I tried using the strtotime() function, but it behaves unpredictably. For example, here's my code:\n\n```php\n$dateString = "2023-12-25 14:30:00";\n$timestamp = strtotime($dateString);\necho $timestamp; // Outputs 1703514600 - seems to work\n```\n\nBut when I pass a date in the format "25.12.2023" or "12/25/2023", strtotime() returns false or an incorrect value. I also noticed that